Principles of Interoperability among GNSS

Within the last decade, several new (and modernized) global and regional navigation satellite systems have been announced. One of the main technical reasons behind this phenomenon is that a single GNSS system is often not enough to guarantee the target user performances, especially in challenging conditions such as urban environments. Therefore the emergence (and modernization) of GNSS systems entails discussions on compatibility and interoperability among the different service providers.

Definition

“Interoperability refers to the ability of global and regional navigation satellite systems and augmentations and the services they provide to be used together to provide better capabilities at the user level than would be achieved by relying solely on the open signals of one system.”

Furthermore, interoperability is often discussed at two different levels: system and signal[1].

Interoperability at System Level

At system level, interoperability can be viewed as the capability of all systems to provide the same solution standalone (with the respective performance constraints). In other words, a GPS, GLONASS or Galileo receiver should be able to provide the same navigation solution – within the respective system accuracy - when used standalone.
In this scope, GPS and Galileo can be said to be interoperable at system level[1], while bringing the advantage of being independently operated thus providing redundancy to the GNSS user community – hence increasing the market confidence on the technology.

Interoperability at Signal Level

Signal interoperability is achieved when the signals provided by different systems are similar enough to allow a GNSS receiver to use those signals with minor modification.
For GNSS, signal interoperability considers the following factors[1]:

Although the international civil coordinate reference standard is the International Terrestrial Reference Frame (ITRF), each GNSS has its own reference frame - which depends on the control stations’ coordinates hence guaranteeing independence among systems. Two GNSS are said to be interoperable from a reference frame perspective if the difference between frames is below the target accuracy. As an example, GPS coordinate reference frame is WGS84 whereas Galileo uses Galileo Terrestrial Reference Frame (GTRF); their difference is expected to be within 3 cm, hence guaranteeing interoperability for most applications.

Time reference frames refer to the international civilian time standard: Universal Time Coordinated/ Atomic Time (UTC/ TAI). Although GPS Time and Galileo System Time (GST) are expected to be within the nanoseconds order of magnitude, the service providers have agreed to broadcast the GPS/ Galileo time offset which can be used at the receiver. As an alternative, some receivers also isolate this time offset as an additional unknown to be solved within the navigation solution. In this context, GPS and Galileo can be considered to be interoperable.

Use of the same carrier frequency

The selection of the same carrier frequency has a high impact on receiver complexity and cost (e.g. it dictates the need for additional band-pass filters). In this scope, GPS and Galileo can be considered interoperable at signal level among themselves in some frequency bands (e.g. L1 and L5/ E5a), but not with the legacy GLONASS signals which use FDMA techniques, hence a different carrier frequency per satellite. Furthermore, please note that even GPS and Galileo may be considered as not interoperable among themselves in frequency-bands that have no correspondence, such as E5b or L2 – even though still compatible since no interference is caused on the other system.

Signals In Space

Aspects of the design of the Signals In Space, such as modulation, signal structure or selection of the codes that require only “software modifications” at the receiver can be considered to not affect interoperability. Furthermore, several working groups have been formed at international level in order to coordinate during the design of the signals design in order to ensure compatibility and signal interoperability. As a consequence, the military GPS-M code and the Galileo Public Regulated Service (PRS) have signal interoperability on L1 band. In addition, QZSS plans to be interoperable with GPS and Galileo both in L1 and E5a/L5 bands.

International Cooperation

GNSS signal design (e.g. signal structures, messages, carrier frequencies, codes and modulations) affect directly interoperability and therefore cooperation at international level has been conducted from an early stage of development in order to guarantee interoperability. Several cooperation venues have been undertaken among different parties, either bilaterally or multilaterally. Some examples:

1998: US-Japan statement on GPS cooperation (QZSS and GPS to be fully compatible and highly interoperable)

Receivers

As a consequence of the emergence (and modernization) of GNSS, GNSS receiver manufacturers claim tend to develop more flexible boards that are able to easily integrate any new system with minor modifications. Currently, there is a wide range of GPS/ GLONASS and SBAS receivers. Furthermore, many manufacturers are already selling Galileo receivers even if the constellation is not yet fully deployed.

Furthermore there are studies showing the advantage of using a modelling language to facilitate the implementation of the navigation message decoding from different GNSS in the receivers, in-line with the respective SIS ICDs[2].